I have been running 400W 10,000K Reeflux bulbs using a PFO HQI ballast for 5 years over my two cube tanks and it is 16" to the sand with the lights mounted 9" off the water surface. For some corals it is too much light but for others it is required to keep the colors I want.
When I say too much light for some corals I mean they are not the colors I'm trying to achieve. SPS corals are quite adaptive and on average darken under low light and lighten when the light is increased. It can take more than six months for corals to adapt to changes in light intensity although the average time is two to four weeks.
Regards,
Kevin